Method of zinc oxide film grown on the epitaxial lateral overgrowth gallium nitride template

ABSTRACT

A growth method is proposed for high quality zinc oxide comprising the following steps: (1) growing a gallium nitride layer on a sapphire substrate around a temperature of 1000° C.; (2) patterning a SiO 2  mask into stripes oriented in the gallium nitride &lt;1  1 00&gt; or &lt;11  2 0&gt; direction; (3) growing epitaxial lateral overgrowth of (ELO) gallium nitride layers by controlling the facet planes via choosing the growth temperature and the reactor; (4) depositing zinc oxide films on facets ELO gallium nitride templates by chemical vapor deposition (CVD). Zinc oxide crystal of high quality with a reduced number of crystal defects can be grown on a gallium nitride template. This method can be used to fabricate zinc oxide films with low dislocation density lower than 10 4 /cm −2 , which will find important applications in future electronic and optoelectronic devices.

This application claims priority to U.S. Provisional Patent ApplicationSer. No. 60/794,775 filed on Apr. 25, 2006, which is herein incorporatedby reference in its entirety.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to the formation of high quality zinc oxide filmsin the fabrication of electronic and optoelectronic devices, and moreparticularly, to the growth of zinc oxide on epitaxial lateral overgrown(ELO) gallium nitride templates.

(2) Description of the Related Art

As a direct band gap semiconductor with a room temperature energy gap of3.37 eV, zinc oxide presents interesting electrical, optical, acousticand chemical properties, which may find wide applications in the fieldsof optoelectronics, sensors and catalysis. With a large exciton bindingenergy (60 meV) [R. D. Vispute, V. Talyansky, S. Choopun, R. P. Sharma,T. Venkatesan, M. He, X. Tang, J. B. Halpern, M. G. Spencer, Y, X. Li,L. G. Salamanca-Riba, A. A. Iliadis and K. A. Jones, Appl. Phys. Lett.73, 348 (1998).] and low power thresholds [D. C. Reynolds, D. C. Look,and B. Jogai, Solid State Commun. 99, 873 (1996).], zinc oxide is alsobeing considered as a promising material for UV and blue light emittingdevices. [M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E.Weber, R. Russo, and P. Yang, Science 292, 1897 (2001); M. Kawasaki, A.Ohtomo, H. Koinuma, Y. Sakurai, Y. Yoshida, Z. K. Tang, P. Yu, G. K. L.Wang, and Y. Segawa, Mater. Sci. Forum 264, 1459 (1998).; D. M. Bagnall,Y. F. Chen, Z. Zhu, T. Yao, S. Koyama, M. Y. Shen, and T. Goto, Appl.Phys. Lett. 70, 2230 (1997).] Epitaxial zinc oxide films have been grownon sapphire by several groups [M. Kawasaki, A. Ohtomo, H. Koinuma, Y.Sakurai, Y. Yoshida, Z. K. Tang, P. Yu, G. K. L. Wang, and Y. Segawa,Mater. Sci. Forum 264, 1459 (1998).; D. M. Bagnall, Y. F. Chen, Z. Zhu,T. Yao, S. Koyama, M. Y. Shen, and T. Goto, Appl. Phys. Lett. 70, 2230(1997).; V. Srikant, V. Sergo, and D. R. Clarke, J. Am. Ceram. Soc.78,1931 (1995).] despite the high mismatch between the two structures.

U.S. Pat. Nos. 5,569,548 and 5,432,397 to Koike et al discuss growingzinc oxide on a sapphire substrate. These patents teach the addition ofnickel, iron, or copper to the zinc oxide to improve latticeorientation. U.S. Pat. No. 5,815,520 to Furushima also teaches growingzinc oxide on sapphire.

Similarly to gallium nitride, zinc oxide has a wurtzite-type crystallinestructure. Vispute et al. have reported the epitaxial growth of zincoxide on gallium nitride. This combination is very interesting since thelattice mismatch between these two materials is as low as 1.9%. However,because of the large dislocation density (˜10⁹ cm⁻²) in the galliumnitride grown on c-sapphire, the as-grown zinc oxide films on galliumnitride are known to contain a high density of defects, which mainlyinclude threading dislocations. Thus, it is important to obtain zincoxide films with high crystalline quality and low dislocation densityfor the realization of high-efficiency zinc oxide devices. U.S. Pat. No.5,679,476 to Uemura et al discloses epitaxially growing a non-defectlayer on a substrate. U.S. Pat. No. 6,274,518 to Yuri et al epitaxiallygrows gallium nitride on a substrate. U.S. Pat. No. 6,673,478 to Kato etal epitaxially grows zinc oxide on a gallium nitride layer. Kato et aluses a growth substrate wherein a plurality of the (0001) surfaces arealigned in a sequence of terraces at an inclination angle of 0.1 to 0.5degree with respect to the growing surface. The quality of Kato's ZnO isnot as high as the quality of the ZnO produced by the process of thepresent invention.

The epitaxial lateral overgrowth (ELO) method relies on selectiveepitaxy and growth anisotropy, which significantly reduces thedislocation density of gallium nitride from 10⁸⁻¹⁰ to 10⁶⁻⁷ cm⁻². [T.Nishinaga, T. Nakano, and S. Zhang, Jpn. J. Appl. Phys. 27 L964 (1988).;T. S. Zheleva, O.-H. Nam, M. D. Bremser, and R. F. Davis, Appl. Phys.Lett. 71, 2472 (1997).] Y. Honda et. al have proposed Facet-ControlledELO (FACELO-through various growth parameters to control the growthfacets) and also successfully reduced the dislocation density to thesame level. [Y. Honda, Y. Iyechika, T. Maeda, H. Miyake and K. HiramatsuJpn. J. Appl. Phys. 40 L309 (2001)] Thus, it is promising to utilize thehigh-quality ELO gallium nitride to obtain zinc oxide films with lowerdefect density. In this invention, an epitaxial growth of zinc oxidefilms using FACELO gallium nitride template on sapphire (0001) isreported. By employing SiO₂ as a mask layer, the selective growth ofzinc oxide films has been realized. Electron microscopy studies show thefilms are single crystalline structures with low dislocation density.Photoluminescence (PL) spectroscopy demonstrates a strong ultraviolet(UV) peak from the zinc oxide. The green emission is also effectivelysuppressed by the high crystalline quality of zinc oxide.

Potential applications of the invention include UV detectors, lightemitting diodes, laser diodes capable of emitting blue and green lightand other optical electronics applications. Other applications alsoinclude transparent conductors, dielectrics and solar cells.

SUMMARY OF THE INVENTION

It is therefore a principal object of the present invention to provide anew method of fabricating a zinc oxide semiconductor epilayer on apatterned gallium nitride template.

It is another object of the invention to provide a method of fabricatinga zinc oxide substrate wafer that is suitable for industrial zinc oxidefabrication.

In accordance with the objects of the invention, a new method offabricating a zinc oxide semiconductor layer is achieved. An underlyinggallium nitride layer is covered with a mask that includes an array ofopenings therein. An overgrown gallium nitride semiconductor layer isformed on the underlying gallium nitride layer through the array ofopenings. Zinc oxide is laterally grown on the overgrown gallium nitridesemiconductor layer to form a continuous overgrown single crystallinezinc oxide semiconductor layer.

Also in accordance with the objects of the invention, an electronic oroptoelectronic device is achieved, comprising: an underlying galliumnitride layer having a predetermined defect density, an overgrowngallium nitride layer contacting the underlying gallium nitride layerthrough an array of openings in a mask wherein (11 22) facets form inthe overgrown gallium nitride layer resulting in a lower defect densitythan the predetermined defect density, a continuous film of zinc oxidelayer overlying the overgrown gallium nitride layer forming a zinc oxidesemiconductor layer, and an optoelectronic or microelectronic device inthe continuous zinc oxide semiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings forming a material part of thisdescription, there is shown:

FIG. 1A illustrates the cross sectional view of an example of a ELO zincoxide semiconductor structure fabricated according to the presentinvention.

FIGS. 1B and 1C show two schematic cross sectional views of theZnO-containing compound semiconductor device according to possibleapplications of the present invention.

FIGS. 2 through 6 illustrate the cross sectional views of eachintermediate fabrication step of the example in FIG. 1.

FIGS. 7A and 7B show the cross-sectional scanning electron microscopy(SEM) and the top view SEM images, respectively, of the zinc oxide/ELOgallium nitride grown for 30 minutes.

FIGS. 7C and 7D show the cross-sectional SEM and the top view SEMimages, respectively, of the zinc oxide/ELO gallium nitride grown for 40minutes.

FIG. 8A is a high resolution transmission electron microscopy (HRTEM)image and the corresponding SAED pattern of zinc oxide/ELO galliumnitride interface.

FIG. 8B is a cross-sectional transmission electron microscopy (TEM)image with g=1 100 near the interface of zinc oxide/ELO gallium nitride.

FIG. 9 is the room temperature micro-PL spectra taken from two differentregions of the zinc oxide/ELO gallium nitride.

FIG. 10 is the X-ray diffraction ω/2θ scan of the epi-zinc oxide/ELOgallium nitride/sapphire (0001) heterostructure.

FIG. 11A shows the AFM of the epi-zinc oxide on the ELO gallium nitridesurface.

FIG. 11B shows the AFM of the as grown zinc oxide on c-plane galliumnitride surface. Inset shows the SEM of the same surface area of the twodifferent samples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Zinc oxide films have been successfully grown on the ELO gallium nitridetemplates of the present invention. The high-quality ELO gallium nitrideis used to obtain zinc oxide films with lower defect density.Furthermore, compared to the conventional single crystalline zinc oxidesubstrate growth by the hydrothermal method, the present invention caneasily be used to get a 2-inch and 3-inch zinc oxide substrate wafer. Assuch, the proposed method of fabrication is also suitable for industrialzinc oxide fabrication.

The proposed method for the growth of zinc oxide films on ELO galliumnitride is described as follows:

A 1-2 μm single crystalline gallium nitride layer grown on anysubstrate, such as sapphire, by any well known method may be used here,such as metal organic chemical vapor deposition (MOCVD). A SiO₂ masklayer is deposited by plasma enhanced chemical vapor deposition (PECVD)at a temperature of about 280° C. on the gallium nitride (0001) surface.Then the SiO₂ mask is patterned into stripes oriented in the galliumnitride <1 100> direction by conventional photolithography. Next, thegallium nitride is re-grown by metal organic chemical vapor deposition(MOCVD) with trimethyl gallium (TMGa) and ammonia (NH₃) used as sourcesfor Ga and N₂ with H₂ as a carrier gas. Next, the ELO galliumnitride/sapphire substrates are put into a tube furnace to grow zincoxide films by chemical vapor deposition and condensation of Zn (99.9%purity) powder in the presence of oxygen.

It is found that the photoluminescence from the zinc oxide films iscentered at 379 nm at room temperature. The luminescence from the zincoxide films is in the UV region, which is suitable for the fabricationof UV LEDs. Further, it is noticed that the intensity of the green bandin zinc oxide PL spectra is very low as shown in FIG. 9B suggesting alow concentration of defects in the fabricated zinc oxide films. This isbecause green emission in zinc oxide is normally ascribed to the oxygenvacancies and/or interstitial zinc ions in a zinc oxide lattice.

The present invention is now described more fully hereinafter withreference to the accompanying drawings, in which embodiments of theinventions are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein. In the drawings, the thicknesses of layersand regions are exaggerated for clarity and are not drawn to scale.

Referring now to FIG. 1A, ELO zinc oxide structures according to thepresent invention are illustrated. The ELO zinc oxide structures 100include a substrate 101. The substrate may be sapphire, silicon, SiC orany other suitable substrates. However, preferably, a sapphire (0001)substrate 101 a is used and a low temperature gallium nitride bufferlayer 101 b is grown on the sapphire substrate 101 a.

The fabrication of substrate 101 is well known to those skilled in theart and need not be described further. An underlying gallium nitridelayer 103 is also grown on the buffer layer 101 b on top of substrate101 a. The underlying gallium nitride layer 103 may be between about 1.0and 2.0 μm thick, and may be formed using heated metal organic chemicalvapor deposition (MOCVD). The underlying gallium nitride layer generallyhas an undesired relatively high defect density, for example dislocationdensities of between about 10⁸ and 10¹⁰ cm⁻². These high defectdensities may result from mismatches in lattice parameters between thebuffer layer 101 b and the underlying gallium nitride layer 103. Thesehigh defect densities may impact performance of microelectronic andoptoelectronic devices in the underlying gallium nitride layer 103.

As shown in FIG. 1A, a mask such as a silicon dioxide mask 105 is formedon the underlying gallium nitride layer 103. The mask 105 includes anarray of openings therein. Preferably, the openings are stripes thatextend along the <1 100> direction of the underlying gallium nitridelayer 103. The mask 105 may have a thickness of about 100 nm and may beformed on the underlying gallium nitride layer 103 using plasma enhancedchemical vapor deposition (PECVD) at about 280° C. The mask 105 may bepatterned using standard photolithography techniques and etched in abuffered hydrofluoric acid (HF) solution.

FIG. 1A also illustrates a {11 22} facets gallium nitride layer 109grown from the underlying gallium nitride layer 103 and through thearray of openings in window area 107 (see FIG. 3). The ELO zinc oxidesemiconductor structure 100 also includes zinc oxide layer 111 a that isgrown by chemical vapor deposition and a lateral zinc oxide layer 111 bthat extends laterally from the {11 22} facets gallium nitride layer109. The lateral zinc oxide layer 111 b may be formed using vapor phaseepitaxy (CVD) as described below. As used herein, the term “lateral”denotes a direction parallel to the faces of substrate 101.

As shown in FIG. 1A, lateral overgrown zinc oxide layer 111 b coalescesat interface 111 a to form a continuous single crystalline zinc oxidesemiconductor layer 111. It has been found that the threadingdislocations in the lateral grown zinc oxide layer 111 will be bent intothe lateral direction even though some threading dislocations willremain and go through the top zinc oxide surface in the window area.Thus, lateral zinc oxide layer 111 b can have a relatively lower defectdensity, for example less that 10⁴ cm⁻². Accordingly, the lateralovergrown zinc oxide layer 111 b is of device quality.

Referring now to FIGS. 2-5, methods of fabricating ELO zinc oxidesemiconductor structures according to the present invention will now bedescribed. As shown in FIG. 2, an underlying gallium nitride layer 103is grown on a substrate 101. The substrate 101 may include a sapphire(0001) substrate 101 a and a low temperature grown gallium nitridebuffer layer 101 b. The low temperature (500° C.˜600° C.) galliumnitride buffer layer 101 b may be deposited on the sapphire substrate101 a in a cold wall vertical and inductively heated metal organicchemical vapor deposition (MOCVD) system up to 30˜40 nm thick. Thegallium nitride layer 103 may be between 1.0 and 2.0 μm thick, and maybe grown at a temperature of at least 1000° C. on the low temperaturegallium nitride buffer layer using any well known method such asmolecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE) andmetal organic chemical vapor deposition (MOCVD).

Referring to FIG. 3, the underlying gallium nitride layer 103 is maskedwith a mask layer 105 that includes an array of openings 107 therein.The mask layer consists of a material (e.g. SiO₂ or SixNy) that does notallow the growth of subsequent gallium nitride that is deposited on it;i.e. selective to the deposition of gallium nitride. That is, GaN willonly grow on the opening area 107 and will not grow on the maskmaterials 105. For example, SiO₂ or SiN can be used for the mask. Themask layer may have a thickness of about 100 nm and may be formed on theunderlying gallium nitride layer 103 using plasma enhanced chemicalvapor deposition (PECVD) at 280° C. The mask 105 may be patterned usingstandard photolithography techniques and etched in a bufferedhydrofluoric acid (HF) solution. Other conventional methods such as dryetching using Reactive Ion etching (RIE) or Inductively Coupled Plasma(ICP) etching may also be used. In one embodiment, the openings 107 are3 μm wide along the <1 100> direction on the underlying gallium nitridelayer 103. The ratio of width of the gallium nitride window area to theSiO₂ wing area can be defined as any value. Prior to further processing,the structure may be dipped in a 50% buffered hydrochloric acid (HCl)solution to remove surface oxides formed on the underlying galliumnitride layer 103.

Referring now to FIG. 4, the underlying gallium nitride layer 103 isgrown through the array of openings 107 to form {11 22} facets F galliumnitride layer 109 grown from the underlying gallium nitride 103 andthrough the array of openings in window area 107. The {11 22} facets Fgallium nitride layer 109 may be grown using MOCVD at about 900-950° C.and with a pressure in the range of 200-500 Torr. Precursors oftrimethylgallium (TMGa) at 80 μmol/min and NH₃ at around 11 slm may beused to form the gallium nitride layer 109. If gallium nitride alloysare formed, additional conventional precursors of aluminum or indium,for example, may also be used. Triethylgallium (TEGa) or ethyldimethylgallium (EDMGa) can also be used as group III precursors, whiledimethylhdrazine ((H₂N₂(CH₃))₂. 1,1DMH_(y)) is preferred as a Nprecursor. The ELO gallium nitride layers are grown by controlling thefacet planes via choosing the growth temperature and the reactorpressure. The typical growth temperature and reactor pressure used forachieving growth on the desired facet planes are 900˜950° C. and apressure with the range of 200-500 Torr, respectively. The originalserrated ELO gallium nitride stripe 109 has a height of 5 μm and a widthof 7 μm, for example.

It is also understood that lateral growth in two directions may be usedto form an overgrown gallium nitride semiconductor layer. Specifically,mask 105 may be patterned to include an array of openings 107 thatextend along either <1 100> or <11 20>. The openings can form stripedpatterns.

Referring now to FIG. 5, the ELO gallium nitride/sapphire substrates areput into a tube furnace to grow zinc oxide films by chemical vapordeposition and condensation of Zn powder in the presence of oxygen. Thecontinued growth of the zinc oxide layer 111 a causes lateral overgrowthof zinc oxide on the underlying gallium nitride layer 109, to formlateral zinc oxide (11 20) facets M. The growth of the zinc oxide layer111 a is formed by chemical vapor deposition and condensation of Zn(99.9% purity) powder in the presence of oxygen. An alumina boat with Znpowder is placed at the center of a quartz tube and purged with Helium(99.999% purity) flowing at a rate of 100 standard cubic centimeters perminute (sccm). The furnace temperature is increased to around 750°C.˜850° C., and oxygen (99.99% purity) flow is introduced to the tubereactor at a flow rate of 10˜100 sccm, and preferably 10˜20 sccm.

The O₂ is mixed with He gas and the flow is maintained throughout thewhole reaction process. Pictures of the zinc oxide layer for 30-minutegrowth and 40-minute growth are shown in FIG. 7.

Referring now to FIG. 6, lateral overgrowth is allowed to continue untilthe lateral growth fronts coalesce at interfaces 111 a, to form acontinuous zinc oxide layer 111. The total growth time may beapproximately 60 minutes. The ZnO film thickness is dependent on thegrowth time. For example, an experimental thickness of 8.9 μm wasachieved after a growth time of 30 minutes.

As shown in FIG. 1B, microelectronic and optoelectronic devices may thenbe formed in regions 111 b. Devices may also be formed in region 111 aif desired.

Examples of ZnO semiconductor devices are described below. FIGS. 1B and1C schematically show two ZnO semiconductor devices according to anembodiment of the present invention. Explanation for elements used inFIGS. 1B and 1C that are identical to those shown in FIG. 1A is omittedby giving similar reference symbols.

In the semiconductor device shown in FIG. 1B, a GaN layer 101 is formedon the sapphire substrate, and thereon the p-type GaN single crystallinetemplates (103 and 109) and the n-type ZnO single crystalline layer 111b are grown sequentially.

The n-type ZnO single crystalline layer 111 b is a ZnO singlecrystalline layer having a film thickness determined by the requirementfor coalescence of the ZnO grown from the GaN ridges. The ZnO is dopedby a group III element such as gallium (Ga) or aluminum (Al) to aconcentration of about 10¹⁸ cm⁻³. Part of the n-type ZnO singlecrystalline layer 111 b is removed to enable the formation of the p-typecontact with GaN 113. A first electrode 112 is formed as the metalcontact to the n-type ZnO.

In order to form ohmic-contact between the n-type ZnO single crystallinelayer 111 b and the first electrode 112, it is preferable that the firstelectrode 112 is formed by, for example, indium (In) and aluminum (Al).

A pn-junction is formed by forming, for example, a p-type GaN layer 103having a thickness of 1 to 4 μm.

A second electrode 113 is formed on a region of the exposed surface ofthe p-type GaN single crystalline layer 103. For making ohmic-contactbetween the p-type GaN single crystalline layer 103 and the secondelectrode 113, metal such as nickel (Ni), platinum (Pt), palladium (Pd),gold (Au), etc., an alloy of two or more of these metals, or amultilayer stack or these metal films is used.

In a semiconductor device as described above, a positive voltage isapplied to the second electrode 113 relative to the first electrode 112,to allow a forward current across the pn-junction. Recombination ofelectrons and positive holes in the region of the p-type GaN 109/n-typeZnO 111 b interface, or the like, produces light emission. It ispossible to use the resulting device as a light emitting diode.

In the semiconductor device shown in FIG. 1C, a GaN layer 103 is formedon the sapphire substrate, and thereon the ZnO 111 b grown on the GaNridges 109 is used as the substrate for the further device structures.The n-type ZnO 114, ZnO/Mg/ZnO quantum well structures 115, and p-typeZnO layers 116 are grown in this order.

The n-type ZnO single crystalline layer 114 is a ZnO single crystallinelayer having a film thickness of 1-4 μm on the surface of the ZnO 111 b.The ZnO is doped by a group III element such as gallium (Ga) or aluminum(Al) to a concentration of about 10¹⁸ cm⁻³. Part of the n-type ZnOsingle crystalline layer 114 is removed to enable the formation of then-type contact. A first electrode 118 is formed as the metal contact tothe n-type ZnO.

In order to form ohmic-contact between the n-type ZnO single crystallinelayer 114 and the first electrode 118, it is preferable that the firstelectrode 118 is formed by, for example, indium (In) and aluminum (Al).

A pn-junction is formed by forming, for example, a p-type ZnO layer 116having a thickness of 1 to 4 μm.

A second electrode 119 is formed on a region of the exposed surface ofthe p-type ZnO single crystalline layer 116. For making ohmic-contactbetween the p-type ZnO single crystalline layer 116 and the secondelectrode 119, metal such as nickel (Ni), platinum (Pt), palladium (Pd),gold (Au), etc., an alloy of two or more of these metals, or amultilayer stack or these metal films is used.

In a semiconductor device as described above, a positive voltage isapplied to the second electrode 119 relative to the first electrode 118,to allow a forward current across the pn-junction. Recombination ofelectrons and positive holes in the region of the quantum wellstructures 115, or the like, produces light emission. It is possible touse the resulting device as a light emitting diode.

In the above, although the crystal-growth substrate, the manufacturingmethod of the ZnO semiconductor crystal, and the ZnO semiconductordevice according to the embodiments of the resent invention areexplained, the present invention is not limited to the embodiments.

EXAMPLE

The following Example is given to show the important features of theinvention and to aid in the understanding thereof. Variations may bemade by one skilled in the art without departing from the spirit andscope of the invention.

FIGS. 7A and 7C show cross-sectional SEM images of zinc oxide filmsgrown on the ELO gallium nitride templates of the present invention for30 minutes and 40 minutes, respectively. In FIG. 7A, it can be seen thatthe original serrated surface of ELO gallium nitride triangular stripehas a height of 5 μm and a width of 7 μm. After zinc oxide growth, thenear rectangle shape is observed with a width of about 6.2 μm,indicating that the significant lateral growth of zinc oxide 111 aoccurred on the ELO gallium nitride 109 and the faster growth facet is(11 20). Furthermore, no growth was found on the SiO₂ mask layer. Thisshows that the zinc oxide top layer was selectively grown on the ELOgallium nitride template. Such morphology originates from the differentgrowth modes between the ELO gallium nitride and c-gallium nitridesurface. FIGS. 7B and 7D show the top view of the sample after zincoxide is grown on the gallium nitride template for 30 mins and 40 mins,respectively. The defect pits (circled) can be found on the surface ofthe top layer, which may come from the threading dislocation propagatingfrom the ELO gallium nitride into the zinc oxide films.

FIG. 8A shows the typical HRTEM image of the zinc oxide/ELO galliumnitride interface, from which it can be seen that the lattice fringes ofzinc oxide are perfectly aligned with those of ELO gallium nitride andthe interface is sharp on the atomic level. The corresponding selectivearea electron diffraction (SAED) pattern is shown in the inset. Only oneset of SAED pattern is observed, resulting from the very close latticematching between zinc oxide and gallium nitride hexagonal structures.The pattern also verifies the perfect epitaxial growth of zinc oxide ongallium nitride and their high crystal quality. A cross-sectional TEMimage with lower magnification is presented in FIG. 8B to further showthe interface of the zinc oxide/ELO gallium nitride. The formation ofthe horizontal dislocations (HD's) is very important due to the factthat HD's can dramatically decrease the threading dislocation (TD's)density of the over grown gallium nitride regions. The image in FIG. 8Bshows that the laterally overgrown zinc oxide is essentially free ofTD's and the HD's lying on the (0001) plane of zinc oxide can beproduced by 90°-bending of TD's in gallium nitride (TD 1). The effect ofbending can be understood by considering the energy of dislocation linesemerging from a free surface of a crystal. [J. P. Hirth and J. Lothe,Theory of Dislocations, 2nd ed. Wiley, New York, (1982)] From the pointof view of the dislocation line tension, any dislocation would tend tobecome perpendicular to a free surface to diminish its energy. As aresult, dislocations would gradually change their line directionstowards the normal direction of the current facet plane, as can be seenin FIG. 8B, which suggests that high quality zinc oxide films can bepseudomorphically grown (along the {11 20} facet) on the ELO galliumnitride. Using these growth conditions, high quality zinc oxideepilayers were fabricated on ELO gallium nitride as shown in FIG. 1A.Here, the HRTEM studies further confirm the suitability of ELO galliumnitride layer for zinc oxide growth.

FIG. 9 shows the PL spectrum (a) obtained from ELO gallium nitride areaI (shown in FIG. 7B), which is mainly contributed by the ELO galliumnitride. FIG. 9 also shows the PL spectrum (b) obtained from the ELOzinc oxide area II (shown in FIG. 7B), which is mainly ascribed to theELO zinc oxide layers. The PL spectra for ELO gallium nitride filmdemonstrate distinct peaks due to the neutral-donor-bound DX excitonemission 91 and free-exciton D₂₀-X transitions with replicas 93. TheD₂₀-X PL peak 91 is mainly caused by the Si donors diffusing from theSiO₂ mask layers via ELO re-growth. The PL peak of the zinc oxide film95 clearly shows the 3.27 eV zinc oxide DX free exciton recombination.From the PL studies, it is noted that full width at half maximum (FWHM)of the zinc oxide peak line width is about 11 meV, which is better thanthe result of 20 meV from heteroepitaxial growth of zinc oxide directlyon gallium nitride. Such small FWHM of the zinc oxide films of thepresent invention is due to their high crystalline quality. It isfurther noticed that the intensity of the green band 97 in zinc oxide PLspectra is very low in FIG. 9 (b), suggesting a low concentration ofdefects in these fabricated zinc oxide films, since the green emissionin zinc oxide is normally ascribed to the oxygen vacancies and/orinterstitial Zn ions in zinc oxide lattice. [e. g. J. Joo, S. G. Kwon,J. H. Yu, T. Hyeon, Adv. Mater. 17, 1873, (2005).] Thus, the method ofthe present invention can be readily used in the growth and fabricationof UV LEDs and LDs.

FIG. 10 shows an X-ray diffraction ′Ω/2θ scan of the zinc oxide filmgrown on the ELO gallium nitride/sapphire (0001). The results show only(000X) family of planes of zinc oxide 101 and gallium nitride 103indicating that the zinc oxide/gallium nitride heterostructure isstrongly c-axis oriented normal to the sapphire (0001) plane. The XRDrocking curve full width at half maximum (FWHM) for the zinc oxide andgallium nitride films was found to be 3 arcmin and 5 arcmin,respectively.

FIG. 11A shows the surface morphology of the overgrowth samplecharacterized by the atomic force microscopy (AFM) as well as that of acontrol sample FIG. 11B grown on c-gallium nitride under the same growthconditions. The root mean squared value of the surface roughness of thelateral overgrown zinc oxide on ELO gallium nitride and the controlsample zinc oxide on c-gallium nitride are 0.40 nm and 3.67 nm,respectively. Atomic steps and terraces were observed from the ELO zincoxide sample. Only a few step terminations in AFM observations weredetected, which indicates the high quality of overgrown ZnO sample. Thesurface pits density of the overgrown gallium nitride sample is morethan 100 times reduced compared with the control sample. These smallpits are thought to be related to mixed screw and edge dislocationswhere the step edges meet. This shows that the ELO zinc oxide growthmethod has an effect on the dislocation behavior in the zinc oxidelayer.

Various articles from scientific periodicals and/or patent literatureare cited throughout this application. Each of such articles is herebyincorporated by reference in its entirety and for all purposes by suchcitation.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

1. A method of fabricating a zinc oxide semiconductor layer comprisingthe steps of: masking an underlying gallium nitride layer with a maskthat includes an array of openings therein; forming an overgrown galliumnitride semiconductor layer on said underlying gallium nitride layerthrough said array of openings; and laterally growing zinc oxide on saidovergrown gallium nitride semiconductor layer to form a continuousovergrown single crystalline zinc oxide semiconductor layer.
 2. Themethod according to claim 1 further comprising: forming at least oneoptoelectronic or microelectronic device in said overgrown zinc oxidesemiconductor layer.
 3. The method according to claim 1 wherein saidmasking step comprises the step of: masking said underlying galliumnitride layer with a mask that includes an array of stripe openingstherein, wherein said stripe openings are orientated along a <1 100>direction of said underlying gallium nitride layer.
 4. The methodaccording to claim 1 wherein said mask consists of a material that doesnot allow the growth of subsequent gallium nitride deposited on it. 5.The method according to claim 4 wherein said mask comprises SiO₂ or SiN.6. The method according to claim 1 wherein said masking step comprisesthe step of: masking said underlying gallium nitride layer with a maskthat includes an array of stripe openings therein, wherein said stripeopenings extend along a <11 20> direction of said underlying galliumnitride layer.
 7. The method according to claim 1 wherein saidunderlying gallium nitride layer includes a predetermined defectdensity, and wherein in said subsequent forming said overgrown galliumnitride layer through said array of openings, (11 22) facets formresulting in a lower defect density than said predetermined defectdensity.
 8. The method according to claim 7 wherein said forming saidovergrown gallium nitride layer comprises the step of growing a (11 22)facets gallium nitride layer using metal organic chemical vapordeposition.
 9. The method according to claim 8 wherein said metalorganic chemical vapor deposition comprises flowing TMGa at about 80μmol/min and ammonia at about 11 slm at a growth temperature of about900 to 950° C.
 10. The method according to claim 7 wherein said formingsaid overgrown gallium nitride layer comprises the step of growing a (1122) facets gallium nitride layer using metal organic chemical vapordeposition, molecular beam epitaxy, or hydride vapor phase epitaxy. 11.The method according to claim 7 wherein dislocations that propagatevertically from said underlying gallium nitride layer through said arrayof openings are bent laterally into said overgrown single crystallinezinc oxide so that said zinc oxide layer is of a lower dislocationdefect density than said predetermined defect density.
 12. The methodaccording to claim 1 wherein said underlying gallium nitride layer isgrown on a substrate comprising sapphire, silicon, SiC or any othersuitable substrate.
 13. The method according to claim 12 furthercomprising: depositing a gallium nitride buffer layer on said substrateat a temperature of between about 500 and 600° C.; and growing saidunderlying gallium nitride layer on said gallium nitride buffer layer ata temperature of at least 1000° C.
 14. A method of fabricating a zincoxide semiconductor layer comprising the steps of: providing anunderlying gallium nitride layer having a predetermined defect density;masking said underlying gallium nitride layer with a mask that includesan array of openings therein; forming an overgrown gallium nitride layerthrough said array of openings wherein (11 22) facets form in saidovergrown gallium nitride layer resulting in a lower defect density thansaid predetermined defect density; laterally overgrowing a zinc oxidelayer using chemical vapor deposition on said overgrown gallium nitridelayer until said zinc oxide layer coalesces to form a continuouslaterally grown single crystalline zinc oxide semiconductor layer; andforming an optoelectronic or microelectronic device in said continuouslaterally overgrown zinc oxide semiconductor layer.
 15. The methodaccording to claim 14 wherein said laterally overgrown zinc oxide layerextends in a <1 100> direction of said underlying gallium nitride layer.16. The method according to claim 14 wherein said laterally overgrowingsaid zinc oxide layer comprises thermal vapor phase epitaxy formed byflowing 10-100 sccm oxygen over a zinc source heated to a temperature inthe range of 750° C.-850° C.
 17. The method according to claim 15comprising the steps of: providing a precursor in vapor formsubstantially comprising of Zn_(A)O_(B) wherein subscript A andsubscript B are any combination of integers; and heating said underlyinggallium nitride layer wherein decomposition of at least some of thevapor at the surface of the substrate leads to deposition of a film ofzinc oxide by chemical vapor deposition.
 18. The method according toclaim 17 wherein said heating comprises heating the surface of said 0substrate to a temperature in the range from 700° C. to 900°C.
 19. Themethod according to claim 14 wherein said overgrowing continues for 30to 60 minutes.
 20. An electronic or optoelectronic device comprising: anunderlying gallium nitride layer having a predetermined defect density;an overgrown gallium nitride layer contacting said underlying galliumnitride layer through an array of openings in a mask wherein (11 22)facets in said overgrown gallium nitride layer resulting in a lowerdefect density than said predetermined defect density; a continuous filmof zinc oxide layer overlying said overgrown gallium nitride layerforming a zinc oxide semiconductor layer; and an optoelectronic ormicroelectronic device in said continuous zinc oxide semiconductorlayer.